THE EVIDENCE OF THE ORGANS OF VISION gl 
of as composed of two ganglia. The external of these is clearly the 
‘ganglion of the retina; its cells form the nuclear layer, the striking 
character of which, and close resemblance to the corresponding layer 
in vertebrates, is shown by Claus’ picture, which I reproduce (Fig. 40). 
The internal ganglion with which the optic nerve is in connection 
contains large ganglion cells, which, to- 
gether with smaller ones, form the gang- 
lionie layer of Berger. 
The most recent observations of the 
structure of the compound retina of the 
crustacean eye are those of Parker, who, 
by the use of the methylene blue method, 
and Golgi’s method of staining, has been 
able to follow out the structure of the 
compound retina in the arthropod on the 
same lines as had already been done for 
the vertebrate. These two methods have 
led to the conclusion that the arthropod 
central nervous system and the verte- 
brate central nervous system are built up 
in the same manner—viz. by means of a 
series of ganglia connected together, each 
ganglion being composed of nerve-cells, 
nerve-fibres, and a fine reticulated sub- 
stance called by Leydig in arthropods 
‘Punktsubstanz,’ and known in verte- 
brates and in invertebrates at the present 
time as ‘neuropil.’ <A further analysis 
resolves the whole system into a combi- 
nation of groups of neurones, the cells 
and fibres of which form the cells and 
fibres of the ganglia, while their dendritic 
head] iG 
ay i | lt 
Fic. 40.—BreonarR CELLS OF 
Nucrtear Layer in RETINA 
OF BRaNCHIPUS. (After 
Cavs.) 
f.or.r., terminal fibre - layer 
of retina; x.l.r.g., bipolar 
cells of the ganglion of the 
retina =inner nuclear layer ; 
mL, Punktsubstanz =inner 
molecular layer ; b.m., base- 
ment membrane formed by 
neurilemma round central 
nervous system. 
connections with the terminations of other neurones, together with 
the neuroglia-cells form the ‘neuropil. As is natural to expect, 
that part of the central nervous system which helps to form the 
compound retina is built up in the same manner as the rest of the 
central nervous system. 
Thus, according to Parker, the mass of nervous tissue which 
occupies the central part of the optic stalk in Astacus is composed 
